Vold was trusting system_server too much and allowed for pretty
much any path in mount()/bindMount() calls for incremental.
This CL adds validation to make sure it's only accessing own
directories. This includes enforcing no symlinks in the paths
Ignore-AOSP-First: security fix
Bug: 198657657
Test: manual
Change-Id: I6035447f94ef44c4ae3294c3ae47de2d7210683a
+ Get rid of an extra string copy in path validation function
Bug: 152349257
Test: atest vold_tests
Change-Id: I03a8cab0dd6abd7d5c9dcbbc2acb651e818e6cd8